The invention will be described in the context of an Automatic Vehicle Identification (AVI) system capable of exchanging data codes between an interrogator(reader) and a transponder(tag). The AVI field is but one environment in which the inventive concepts described herein can be applied. Systems using batteryless transponders or transponders with batteries may be used for identifying or locating objects bearing the transponders such as cattle, luggage, manufactured goods, or other items. Further, the transponder might provide status information regarding the object on which it is located, such as a transponder disposed on a car door indicating whether the car door is open. Transponders utilized in the above recognition systems or others may be powered from batteries or from wireless radio frequency (RF) signals.
With respect to AVI systems, generally the interrogator is provided in a toll booth of a toll road, parking garage or other limited access facility. The interrogator(reader) identifies passing automobiles by sending wireless interrogation signals to a transponder (tag), which would normally be a small, self-contained unit placed, for example, on the dashboard or windshield of the car. In this way the car (or other vehicle or object) can be identified in a speedy and efficient manner. Depending on the use of the system, an account associated with the driver, owner or other designated person can be debited an access charge. Compatibility standards for one such AVI system is set out in Title 21, Division 2, Chapter 16, Articles 1-4 of the California Code of Regulations, herein known as the Caltrans specification or Caltrans spec.
With respect to the specific embodiment, which is compatible with the Caltrans spec, the minimum role of the interrogator is to: 1) trigger or activate a transponder; 2) interrogate a transponder for specific information; and 3) provide an acknowledgment message to the transponder after a valid response has been received by the interrogator. The immediate mandate of the Caltrans spec covers electronic toll collection, sometimes described as part of "Electronic Tolls and Traffic Management" (ETTM). The AVI equipment for toll collection will typically consist of two functional elements: vehicle-mounted transponders and fixed position interrogators.
A toll collection site will consist of at least one interrogator operating in the role described above. Upon interrogating or "polling" a transponder for specific information such as a transponder identification (ID), the interrogator (or separate computer) will typically check the transponder ID against a database of valid, non-delinquent accounts. If the transponder ID is valid and not delinquent, the interrogator will send a signal to a gate mechanism, or a toll site computer operating such a gate mechanism to allow the car to pass. Of course other enforcement means are possible which may allow for less interruption of traffic, such as allowing all cars to pass and identifying the auto carrying the transponder (or the rouge automobile carrying an inoperable transponder or no transponder at all) by other means and notifying an appropriate enforcement agency.
The interrogation signal and response signal comprise data codes. The Caltrans spec has set forth definitions for data codes to be transmitted between the interrogator and the transponder. The data codes described below are derived from the Caltrans spec and are merely exemplary and intended to be neither an exhaustive nor a mandatory list of codes for a general AVI system.
(a) Agency Code: This 16-bit code field identifies the agency that has the authority to conduct this transaction; PA1 (b) Error Detection Code: The error detection code may be CRC-CCITT-16, with a generator polynomial of X+X+1. This results in a 16-bit error detection code transmitted with each data message; PA1 (c) Header Code: The Header is generally the first field in each data message for either reader or transponder transmissions and consists of an 8-bit and a 4-bit word for a total of 12 bits. The Header provides a "selsyn" signal that may be used by a receiver with a transponder or interrogator to self-synchronize (selsyn) with the data being received from the interrogator or transponder, respectively. An exemplary selsyn signal might be the binary and hexadecimal values: 10101010 and AA respectively; PA1 (d) The Header Flag code provides for a unique, 4 bit Flag that is recognized by a transponder or interrogator decoder as the end of the Header with the data message to follow. The exemplary Flag signal has binary and hexadecimal values: 1100 and C respectively; PA1 (e) Interrogator ID Number: This 32-bit field is used to uniquely identify the interrogator conducting the transaction; PA1 (f) Transaction Record Type Code: This 16-bit code uniquely identifies a specific type of valid transaction between a reader and a transponder. This code uniquely defines the transponder message fields and functions permissible. By way of example, hexadecimal numbers 1 through 7FFF may be set aside for transponder message structures and 8000 through FFFF may be dedicated for reader-to-transponder message structures; PA1 (g) Transaction Status Code: Used to provide status information to the transponder; and PA1 (h) Transponder ID Number: This 32 bit code uniquely identifies which transponder is responding to a polling request or is being acknowledged.
Because the transponder typically either derive their operating power from a small battery, or from a received Radio Frequency (RF) signal, the transponders are not normally active. Instead, the interrogator will transmit an RF trigger pulse to activate (turn-on) the transponders in approaching cars or other objects. The interrogator may transmit a number of RF trigger pulses at regular intervals to wake up any approaching transponders. Alternatively, the interrogator might send an RF trigger pulse in response to an external stimulus to the interrogator indicating that a transponder is approaching (e.g. light, heat, or magnetic sensors). After a time delay, the reader then will transmit an encoded signal, referred to as the Polling message or interrogation which, upon detection and decoding by the transponder, will provide initial information to the transponder as to which data blocks the transponder should transmit.
In a described embodiment, the interrogator transmits an unmodulated continuous wave RF signal as an interrogation signal to the transponder while waiting for the transponder response signal. BY analog to acoustic signals, an unmodulated RF signal is similar to a constant or "pure" musical tone without any variation in amplitude or frequency. However, it should be mentioned that a signal could be considered "unmodulated" in amplitude even if varying in frequency and vice-versa. The transponder response signal in this embodiment comes when the transponder backscatter modulates the continuous wave RF signal with information from the transponder. Following the acoustic analogy, backscatter modulation is similar to the phenomenon achieved by singing into a fan and listening to the resulting sound. Typically when a person sings, they control the variations or modulations of their voice. Similarly, an RF transmitter is generally able to modulate its signal. However, when a person sings into a fan, the blades of the fan will reflect the sound of the voice immediately back to the person when the blades pass immediately in front of his mouth. Thus, the singer hears a chopping sound superimposed on his voice. That "chopping" sound the signer hears is nothing more than the amplitude variation of the reflection of the sound of his voice. Similarly, the transponder can modulate (by amplitude or other means) the continuous wave RF signal transmitted by the interrogator and this reflected signal will have modulations superimposed on it.
Some of the problems encountered in a toll tag system are only exemplified when the toll booth comprises a canopy which overhangs the toll plaza. The canopy presents a perfect reflective medium for any transponder response or interrogation signal which is transmitted from either the transponder or the interrogator respectively. Furthermore, for automatic toll collection systems, it is essential that a tag responds only in a certain area (i.e. time window) of a toll lane, in order to perform the billing process properly. For example, if a tag responds too early due to reflections off other objects present in the toll lane, the ETC lane sensor sequencing will be messed up, and the wrong car will be billed with the wrong toll transaction which is absolutely not allowed in a toll application.
In a canopy toll booth situation, car 2's tag can be read when the antenna boresight signal (the strongest in the antenna pattern) bounces off vehicle 1's roof. Just lowering the transmitter power to prevent this problem is not enough, because the antenna lane coverage (at the lateral lane edges) might become jeopardized in that case. Also an "RF focusing" effect (depending on road and canopy shape) might be present at a toll plaza, making the reflected signal at car 2 even stronger than at car 1, no matter what power interrogation signal is transmitted.
One prior art solution to the problem of "early read" comprises the use of an extra lane sensor which turns the reader on ONLY when the car is really under it. However, this method is not very reliable because reflection reads are still possible and by turning the reader on ONLY when the car is really under it, the READ zone is detrimentally limited increasing the risk of a tag not being read at all i.e. a missed tag. A second prior art method of early-read prevention comprises designing a method which measures the time delay between reader-transmitted and tag-reflected signals, having an acceptable predetermined time delay yield a properly timed transponder response. However, this method is very costly and needs an averaging scheme at these short distances.